Structure of PtnNi Nanoparticles Electrocatalysts Investigated by X-ray

Article pubs.acs.org/JPCC

Structure of PtnNi Nanoparticles Electrocatalysts Investigated by X‑ray Absorption Spectroscopy Hongliang Bao,†,‡ Jiong Li,† Luhua Jiang,§ Mingfeng Shang,† Shuo Zhang,† Zheng Jiang,† Xiangjun Wei,† Yuying Huang,*,†,‡ Gongquan Sun,*,§ and Jian-Qiang Wang*,†,‡ †

Shanghai Synchrotron Radiation Facility, and ‡Key Laboratory of Interfacial Physics and Technology, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, P.R. China § Division of Fuel Cell & Battery Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P.R. China S Supporting Information *

ABSTRACT: Carbon supported Pt−Ni nanoparticles (NPs) electrocatalysts with nominal Pt/Ni atomic ratios of 3:1, 2:1, and 1:1, denoted as Pt3Ni/C, Pt2Ni/C, and Pt1Ni/C, respectively, were obtained by a modified polyol process. The structure of PtnNi/C (n = 3, 2, 1) electrocatalysts was studied by using an X-ray absorption spectroscopy technique combined with X-ray diffraction and transmission electron microscopy. Significantly, the NPs of Pt3Ni/C in air are demonstrated to have a quasi core−shell structure consisting of a core of metallic Pt surrounded by many small NiOx clusters. In contrast, both Pt2Ni/C and Pt1Ni/C are demonstrated to have an alloy structure with partial oxidation on the surface. Under the atmosphere of H2 at 393 K, the PtnNi/C became the expected bimetallic alloy. At last, we discuss the effect of Ni amount on the structure of PtnNi/C and estimate their possible catalytic activity for methanol oxidation reaction. Our results further confirmed that the structure of Pt−Ni NPs can be influenced by Ni amount and this effect may be enlarged by environment effects.

1. INTRODUCTION Platinum-based bimetallic nanoparticles (NPs) as electrocatalysts exhibit enhanced activity for methanol oxidation reaction (MOR)1−4 in low-temperature direct methanol fuel cells (DMFCs) compared with pure Pt. The promoting effect induced by the second metal may occur through various phenomenon, such as “electronic effects”,5 which refer to changes in electronic structure of Pt atoms on the topmost surface, and “geometric effects”,5−7 which refer to formation of surface oxygenated species on the second metal and changes in geometric structure of Pt ensembles. The Pt−Ni bimetallic NPs have been proposed both as cathode material with improved performance for oxygen reduction reaction (ORR)8,9 and as anode material with improved activity for MOR.10−12 For MOR, it is widely accepted that the most significant reactions are the dissociative adsorption of methanol and the oxidation of CO. Platinum is the most active metal for dissociative adsorption of methanol to intermediates, such as CO, however, as it is well-known, at moderate temperatures it is easily poisoned by the adsorbed COads. Therefore, an improved CO oxidation removal process as well as decreased CO adsorption strength may result in an improved MOR rate. According to “geometric effects”, the surface Ni oxygenated species and unalloyed NiO species may offer OH species to remove the intermediate COads.13,14 According to “electronic effects”, when alloying with Pt, Ni may © 2013 American Chemical Society

modify the electronic structure of Pt and decrease CO adsorption strength.15 Generally, there are (more or less) Ni oxygenic species on surfaces of a Pt−Ni alloy, so the activity enhancement can be better understood by both “geometric effects” and “electronic effects”.16−18 On the other hand, the dissociative adsorption of methanol requires the existence of several adjacent Pt atoms,19 so the presence of the Ni atoms may block methanol adsorption on Pt sites due to the dilution effect. Furthermore, the influences of Ni amount on the MOR performance have been reported.14 The Pt3Ni NPs specifically showed better activity enhancement, and similar phenomenon was observed in other Pt-M (M = Fe, Co, etc.) bimetallic NPs systems.20,21 Our previous work22 also showed that Pt3Ni had a higher electrocatalytic specific activity than both Pt2Ni and Pt1Ni. However, the reports about clear structure−property correlation for the Pt-M bimetallic NPs with varying M amounts are deficient. Although many different techniques, such as TEM and XRD, are well used to characterize Pt-M bimetallic NPs,23,24 it is still a challenge to determine the structure of Pt-M bimetallic NPs because their structures may become complicated by many influencing factors, such as the Received: May 15, 2013 Revised: August 19, 2013 Published: September 12, 2013 20584

dx.doi.org/10.1021/jp404799d | J. Phys. Chem. C 2013, 117, 20584−20591

The Journal of Physical Chemistry C

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Figure 1. (a) TEM image of the Pt3Ni/C catalyst (scale bar = 50 nm) and (b) HRTEM image of the Pt3Ni/C catalyst (scale bar = 5 nm), inset is the magnification of the edge particle (scale bar = 2 nm).

size of NPs,25 amount of the second metal,26−28 synthesis methods,29−32 and treatment protocols,33−36 etc. In addition, the surface atoms are sensitive to the environment,37−40 such as oxygen adsorption being able to dramatically change the surface composition of a Pt−Ru alloy where Ru is brought to the surface from the bulk by adsorbed oxygen and forms island structures.41 X-ray absorption spectroscopy (XAS), containing a near-edge X-ray absorption spectroscopy (XANES) region and an extended X-ray absorption fine structure (EXAFS) region, is a useful tool that can provide complementary information about the electronic structure and the local geometric structure of the absorption atoms. It has been used for structure studies of Pt-M bimetallic systems42−48 such as determining the extent of alloying or atomic distribution in models of bimetallic NPs.49 Nevertheless, XAS generally gives the average information for an ideal alloy without distinguishing the surface and bulk. The atomic distribution of the surface and the morphology of considered surface species are mainly responsible for the MOR behavior of the Pt-M NPs. When the surface atoms have different kinds of coordination atoms in comparison with the bulk atoms, it is possible to get surface and bulk information separately by XAS analysis, which is more valuable in in situ XAS. In this work, we synthesized Vulcan XC72 carbon supported Pt−Ni NPs electrocatalysts with nominal Pt/Ni atomic ratios of 3:1, 2:1, and 1:1, which are denoted as Pt3Ni/C, Pt2Ni/C, and Pt1Ni/C, respectively. The structure of NPs of the PtnNi/C (n = 1, 2, 3) electrocatalysts was studied by using XAS, XRD, and TEM techniques. More detailed structures including some valuable surface information were obtained by systematic XAS analysis. At last, we discussed the effect of Ni amount on the structure of PtnNi/C and estimated their possible catalytic activity for MOR. Our results further confirmed that the structure of Pt−Ni NPs can be influenced by the amount of Ni, and this effect may be enlarged by the environment.

electrocatalysts with nominal Pt/Ni atomic ratios of 3:1, 2:1, and 1:1, which are denoted as Pt3Ni/C, Pt2Ni/C, and Pt1Ni/C, respectively. A similar weight (40 wt %) Pt/C catalyst was also prepared by a similar process for comparison. 2.2. XRD and TEM Characterizations. The powder X-ray diffraction data were collected on Beamline BL14B1 at the Shanghai Synchrotron Radiation Facility (SSRF). The electron storage ring was operated at 3.5 GeV and the wavelength of Xrays employed is 1.2398 Å. The data were collected from 15° to 70° 2θ at a scan rate of 0.02° 2θ per step and 1 s per point. TEM and high-resolution TEM (HRTEM) images were obtained using a FEI Tecnai G2 F20 S-TWIN TEM, equipped with an energy-dispersive X-ray (EDX) detector, operated at an accelerating voltage of 200 kV. The EDX line-scanning data were collected under the scanning transmission electron microscopy (STEM) mode. TEM grids were prepared by loading the as-prepared samples ultrasonically dispersed in ethanol onto a copper grid and drying in air. 2.3. XAS Measurements. The X-ray absorption spectroscopy data were collected on Beamline BL14W1 at the Shanghai Synchrotron Radiation Facility (SSRF). A double Si (111) crystal monochromator was employed for energy selection. High-order harmonics were successfully inhibited using a harmonic suppression mirror. Two gas-filled ionization chambers were used to measure the intensities of the incident beam (I0) and the transmitted beam (It) after the sample. XAS data of the electrocatalysts were acquired in either transmission or fluorescence mode. The gas used in the chambers depended upon the element being examined. For the Ni K-edge, pure N2 was used in both the chambers. For the Pt L3-edge, the ionization chamber before the sample was filled with 100% N2, and the second chamber after the sample was filled with a mixture of 50% N2 and 50% Ar. In the case of fluorescence mode, a Lytle detector was used for collecting the fluorescence information. A Co filter with three absorption lengths was used to reduce the background. XAS data of standard samples, such as Pt foil, Ni foil, and NiO powder, were also measured in a similar condition for comparison. An in situ cell was employed for collecting Ni K-edge XANES data for PtnNi/C catalysts in an atmosphere of 10% H2 and 90% He at different temperature. The in situ XANES data were collected by using a fourelements Si draft detector (SDD). Prior to the XANES measurements, the sample in the cell was treated with a mixed

2. EXPERIMENTAL SECTION 2.1. Preparation of Electrocatalysts. Carbon supported PtnNi (n = 3, 2, 1) electrocatalysts were synthesized by a modified polyol process as described in previous papers.22,50 The H2PtCl6·6H2O and Ni(NO3)2·6H2O were used as precursors and Vulcan XC-72 carbon was used as support. By adjusting the amount of the metal precursors, we synthesized 20585



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gas of 10% H2 and 90% He for 1 h at a flow rate of 30 mL/min. The measurements on each sample was first made at 310 K and then at 393 K twice. 2.4. XAS Data Analysis. XAS data analysis were carried out by using the software package of Ifeffit.51 Standard procedures were followed to analyze the XAS data. First, the raw absorption spectrum in the pre-edge region was fitted to a straight line, and the background above the edge was fitted with a cubic spline. The EXAFS function, χ(E), was obtained by subtracting the postedge background from the overall absorption and then normalized with respect to the edge jump step. The normalized χ(E) was transformed from energy space to k-space, where k is the photoelectron wave vector. The χ(k) data were multiplied by k3 to compensate for the damping of EXAFS oscillations in the high k-region. The range of k3weighted χ(k) data in the k-space using for Fourier transformation (FT) to r-space, and the range of the r-space used for the curve fitting of EXAFS data were compiled in Supporting Information, Table S1. The backscattering amplitude and the phase shift were obtained from theoretical calculation by using the FEFF code (version 6.0).52 An S02 value of 0.80, 0.85, and 0.95 was from fitting of Pt foil, Ni foil, and NiO, respectively. From the analysis, structural parameters, such as coordination numbers (N), bond distance (R), the Debye−Waller factor (σ2), and inner potential shift (ΔE0) have been calculated.

of (111) faces of Pt. It should be note that the corresponding diffraction peaks of PtnNi/C shift to higher angles to some extent compared with Pt/C, which can be attributed to some contract of the lattice constant. The lattice constants of Pt for the Pt/C, Pt3Ni/C, Pt2Ni/C, and Pt1Ni/C catalysts estimated by Vegard’s law are 3.932, 3.920, 3.861, and 3.814 Å, respectively. The average metal particle sizes of the Pt/C, Pt3Ni/C, Pt2Ni/C, and Pt1Ni/C, calculated through the Scherrer equation by analyzing the Pt(111) peak, are 3.7, 2.8, 3.2, and 3.4 nm, respectively. The calculation results match well with the values measured from the TEM images (Supporting Information, Figure S2). Because the nanoparticle sizes are nearly identical for all the catalysts, the influence of the particle size on the structures and the catalytic activities of the PtnNi/C could be greatly reduced. To get further insight on the structure of these catalysts, we performed XAS measurements at room temperature (RT). The Pt L3-edge and Ni K-edge XANES data are shown in Figure 3.

3. RESULTS AND DISCUSSION 3.1. Structure of PtnNi/C Electrocatalysts. The asprepared Pt/C and PtnNi/C electrocatalysts were characterized by TEM and powder XRD techniques. TEM images of Pt/C and PtnNi/C are shown in Figure 1a and Supporting Information, Figure S1, the metal NPs of Pt/C and PtnNi/C catalysts are well dispersed on carbon support with uniform size of approximately 3 nm. Powder XRD profiles of Pt/C and PtnNi/C are provided in Figure 2. The diffraction peaks at

Figure 2. XRD patterns of the Pt/C (black), Pt3Ni/C (red), Pt2Ni/C (blue), and Pt1Ni/C (dark cyan).

Figure 3. (a) Pt L3-edge XANES spectra for Pt/C (olive), Pt3Ni/C (red), Pt2Ni/C (magenta), Pt1Ni/C (blue), and reference of Pt foil (black), with an inset about the magnification of their white lines. (b) Ni K-edge XANES spectra for Pt3Ni/C (red), Pt2Ni/C (blue), Pt1Ni/ C (olive), and references of NiO (black) and Ni foil (magenta).

about 31.7, 36.7, 53.1, and 63.2° could be assigned to the Pt(111), Pt(200), Pt(220), and Pt(311) diffraction indexes, indicating a face-centered cubic (fcc) structure of Pt for the Pt/ C catalyst. Compared with Pt/C, no other diffraction peak in the XRD patterns of Pt nNi/C is observed, indicating predominantly metallic Pt with fcc structure, which is also implied by the appearance of diffractive stripes in HRTEM images (Supporting Information, Figure S1). The spacing of diffractive stripes in HRTEM image of the Pt3Ni/C catalyst (Figure 1b) is about 0.224 nm, such value reflects the interval

Figure 3a shows that the Pt L3-edge XANES of PtnNi/C are similar to that of Pt foil. In contrast, the Ni K-edge XANES of the PtnNi/C are apparently different from that of Ni foil (Figure 3b). These findings reveal the predominantly metallic state of Pt in PtnNi/C, which is consistent with the former XRD and TEM results and is further confirmed by later EXAFS study. It should be noted that the Pt/C and PtnNi/C catalysts 20586



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consistently exhibit increasing white line relative to Pt and Ni foil (at around 11567 eV and 8350 eV, respectively), indicating some degree of oxidation. For Pt3Ni/C, significantly, the appearance of a weak pre-edge peak at 8333 eV (Figure 3b) indicates that most of Ni are existent in the form of NiO oxide or Ni oxidation state.53 In addition, because the peak at 8365 eV is related to the Ni−O−Ni interaction,54 the appearance of a very weak peak at 8365 eV is indicative of the existence of many small Ni oxide clusters, this result is further confirmed by later EXAFS study. Local geometric structure of Pt and Ni atoms for PtnNi/C were obtained by simultaneously analyzing the Pt L3-edge and Ni K-edge EXAFS data. Magnitudes of Fourier transform (FT) to the EXAFS data at Pt L3-edge are shown in Figure 4b. The

Figure 5. (a) k3-weighted Ni K-edge experimental χ(k) data (black) and fit (red) in k-space as well as (b) the corresponding k3-weighted Fourier transform for PtnNi/C catalysts.

contributions was used to fit Pt2Ni/C and Pt1Ni/C data and fine results were obtained finally (Figure 4). For Ni K-edge, both Pt2Ni/C and Pt1Ni/C data were well fitted by using Ni− O, Ni−Ni, and Ni−Pt coordination shells. However, the Pt3Ni/ C data was successfully fitted to a model containing Ni−O, Ni− O−Ni, and Ni−O−Pt contributions (Figure 5). It is similar to that of MoOx@Pt core−shell NPs, the Pt−Mo interactions were not acquired in Pt L3-edge EXAFS fits.55 The Pt2Ni/C and Pt1Ni/C data from both metal edges were fitted simultaneously. The coordination numbers (CNs) of Pt−Ni (NPtNi) and Ni−Pt (NNiPt) were fixed according to eq 1.56 χ NPtNi = Ni NNiPt χPt (1)

Figure 4. (a) k3-weighted Pt L3-edge experimental χ(k) data (black) and fit (red) in k-space as well as (b) the corresponding k3-weighted Fourier transform for Pt/C and PtnNi/C catalysts.

where χNi and χPt were determined by inductively coupled plasma (ICP). The actual atomic ratio of Pt/Ni for Pt3Ni/C, Pt2Ni/C, and Pt1Ni/C are 2.95:1, 1.93:1, and 1.12:1, respectively. The heterometallic bond distances and corresponding Debye−Waller factors were also constrained to be the same. The obtained values of the structure parameters from fitting these EXAFS data are given in Table 1. The results show that the Pt−Pt bond length (RPtPt) of Pt/C, Pt3Ni/C, Pt2Ni/C, and Pt1Ni/C is 2.759 ± 0.003, 2.754 ± 0.004, 2.729 ± 0.004, and 2.729 ± 0.002 Å, respectively, and the trend of the contraction of RPtPt is well consistent with the shift of corresponding diffraction peaks in Figure 2. The significant contraction of RPtPt implies an obvious alloying between Pt and Ni in both Pt2Ni/C and Pt1Ni/C. According to the value of RPtPt, the interval of (111) faces of Pt3Ni/C is 0.225 nm which is well consistent with the result of HRTEM. The NPtPt for PtnNi/C are close to

main symmetrical peaks at about 2.7 Å for Pt/C and Pt3Ni/C result from Pt−Pt contribution, in contrast, the intensive asymmetric peaks at about 2.6 Å for Pt2Ni/C and Pt1Ni/C arise from both Pt−Pt and Pt−Ni interactions. The r-space at Ni Kedge (Figure 5b) show that the PtnNi/C has two main peaks, the first main peak at about 1.7 Å was ascribed to Ni−O contribution. The second main peak for Pt3Ni/C at about 2.6 Å was ascribed to Ni−O−Ni contribution while the asymmetric peak for Pt2Ni/C and Pt1Ni/C at about 2.4 Å was ascribed to both Ni−Pt and Ni−Ni contributions. It should be noted that there is a weak peak at about 3.2 Å, which may be ascribed to Ni−O−Pt contribution. The approach taken in fitting the EXAFS data was to select the simplest model first and attempt to fit the data. According to the above analysis and results, for Pt L3-edge, a two-shell model including Pt−O and Pt−Pt interactions was chosen to fit Pt/C and Pt3Ni/C data while a three-shell model consisting of Pt−O, Pt−Ni, and Pt−Pt 20587



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Table 1. Structural Parameters Obtained from the Analysis of the EXAFS Spectra at Pt L3-Edge and Ni K-Edge for PtnNi/C, Pt/ C Catalysts, and NiO sample NiO Pt/C Pt3Ni/C

Pt2Ni/C

Pt1Ni/C

a

shell

N

Ni−O Ni−O−Ni Pt−O Pt−Pt Pt−O Pt−Pt Ni−O Ni−O−Ni Ni−O−Pt Pt−O Pt−Ni Pt−Pt Ni−O Ni−Ni Pt−O Pt−Ni Pt−Pt Ni−O Ni−Ni

6 (theory) 12 (theory) 1.5 ± 0.4 7.5 ± 0.6 2.2 ± 0.9 7.0 ± 0.9 4.1 ± 0.7 2.4 ± 1.1 2.0a 0.6 ± 0.3 1.3 ± 0.5 7.1 ± 0.9 1.3 ± 0.2 0.5 ± 0.3 0.8 ± 0.2 1.3 ± 0.3 7.4 ± 0.5 1.8 ± 0.6 0.7 ± 0.6

R (Å) 2.084 ± 2.948 ± 1.991 ± 2.759 ± 2.013 ± 2.754 ± 2.045 ± 2.989 ± 3.340 ± 1.997 ± 2.644 ± 2.729 ± 2.064 ± 2.571 ± 2.008 ± 2.640 ± 2.729 ± 2.064 ± 2.587a

0.015 0.004 0.010 0.003 0.013 0.004 0.006 0.008 0.027 0.019 0.013 0.004 0.011 0.014 0.011 0.007 0.002 0.012

σ2 (×10−3 Å2)

ΔE0 (eV)

5.8 ± 1.3 6.0 ± 0.5 6.0 ± 4.0 5.4 ± 0.6 11.1 ± 7.2 5.9 ± 0.9 5.5 ± 1.5 6.9 ± 3.0 8.5 ± 3.0 3.2 ± 6.2 7.0 ± 3.0 6.4 ± 0.7 3.5a 3.6 ± 3.9 7.8 ± 4.6 7.9 ± 1.6 7.1 ± 0.4 4.7 ± 2.9 5.4 ± 5.4

−5.4 ± 3.1 −7.6 ± 0.7 8.7 7.0 10.5 7.4 −2.8 −8.6 −17.4 11.0 5.2 5.3 4.4 −7.0 11.0 5.2 5.5 3.0 −21.3 ± 6.8

This value is set to get a better fitting.

reduced at 393 K, confirming that the oxidation mainly occurs on the surface of NPs. The reduction of Ni oxide in PtnNi/C at lower temperature may be facilitated by the presence of Pt57,58 and the small size of NPs.59,60 Similar phenomenon was reported that Ni species in the Pt−Ni catalysts were completely reduced to the metallic state in H2 at 423 K.61 Energy dispersive X-ray (EDX) line-scanning was performed to confirm the different structures of these catalysts. The EDX line profiles of elements Pt and Ni in the NPs of PtnNi/C catalysts are shown in Supporting Information, Figure S4. For Pt3Ni/C, the Ni traces have two indistinct peaks in two sides while the Pt traces have one peak in the center, indicating the formation of incomplete Ni shell. In contrast, for both Pt2Ni/C and Pt1Ni/C, the Ni traces are similar to Pt traces having one peak in the center, confirming the formation of Pt−Ni alloy. 3.2. Effect of Ni Amount on Structure of PtnNi/C and Their Possible Activity for MOR. Our results show that the as-prepared carbon supported Pt−Ni NPs with different specific Ni amounts have different structures. In air, the NPs of the Pt3Ni/C catalyst have a quasi core−shell structure consisting of a core of metallic Pt surrounded by small NiOx islands and both Pt2Ni/C and Pt1Ni/C catalysts are alloy with partial oxidation on the surface (Figure 6). The Ni atoms in Pt−Ni bimetallic NPs steadily exist in two ways: combining with O and alloying with Pt. The Ni amount may affect the balance of such two ways and finally result in different

that of Pt/C, indicating that predominantly metallic Pt occurred in PtnNi/C. Moreover, the total CNs of Pt are all about 9, close to 10.5 which is computed from a spherical Pt metal particle with a 3 nm diameter (assuming its surface is covered by (111) face). This finding is consistent with the 3 nm average size from TEM images since there are other crystal planes and defects in a real particle. It should be note that the Pt2Ni/C and Pt1Ni/C have a close NPtNi about 1.3. In contrast, the Pt3Ni/C has no observed Pt−Ni interaction. These results indicate that the NPs in both Pt2Ni/C and Pt1Ni/C catalysts are alloy structure while most of Pt atoms occur in Pt3Ni/C in the form of Pt metal. The Pt3Ni/C sample has no observed Nimetal interactions and its NNiO is 4.1 ± 0.7, indicating that most of Ni atoms are combined with oxygen. Moreover, the bond lengths of Ni−O and Ni−O−Ni are close to those of NiO powder. The CN of Ni−O−Ni is only 2.4 ± 1.1, which is much lower than that for the NiO standard sample. These findings confirm that there are many small NiOx clusters in the Pt3Ni/C catalyst. The small values of NPtO and NNiO imply that there is some degree of oxidation of the surface atoms in NPs of Pt/C and PtnNi/C, which is in agreement with the XANES analysis. The NPtO of Pt3Ni/C is larger than that of Pt/C and may be caused by the interaction of NiOx clusters and Pt atoms, which can be implied by the existence of the Ni−O−Pt coordination shell. According to above results and discussions, it demonstrated that the NPs of Pt2Ni/C and Pt1Ni/C catalysts are alloys with predominantly metallic Pt and those of Pt3Ni/C are composed of metallic Pt and small NiOx clusters with ignored interaction of Pt with Ni. However, the distribution of the NiOx clusters in NPs is not clear though they are very possible to disperse on surfaces of the metallic Pt. If these small NiOx clusters distribute on the surface of the Pt core, they may be very active and be easily reduced by H2 at relative low temperature. We therefore implemented the Ni K-edge XANES scans in an atmosphere of 10% H2 and 90% He at different temperature (Supporting Information, Figure S3). The results show that the PtnNi/C can be reduced by H2 at 310 K and completely

Figure 6. The 3D schematic diagrams of the NP for Pt3Ni/C (left), Pt2Ni/C (middle), and Pt1Ni/C (right) with the surface O atoms omitted. The gray, orange, and rose balls represent Pt, Ni, and O atoms, respectively. 20588



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worse Pt surfaces than Pt2Ni/C and may show lower activity for MOR. However, for the Pt3Ni/C, ignoring Pt−Ni interaction, the surface of the Pt core is little disturbed by Ni, though about 20% of the surface is covered by small NiOx clusters (assuming that each NiOx cluster is composed of an average of three layers of Ni−O). According to above discussions, the possible activity for MOR may be Pt3Ni/C > Pt2Ni/C > Pt1Ni/C. Our previous work22 showed that the specific activity for MOR follows the order of Pt3Ni/C > Pt2Ni/C > Pt/C > Pt1Ni/C. Because the structures of NPs are sensitive to environment, the structure of PtnNi/C in electrochemical working conditions and the relationship between structure and MOR performances will be studied in our future work. There is no power evidence to support “electronic effects”. Although the Pt L3-edge white lines of both Pt2Ni/C and Pt1Ni/C are lower than that of Pt/C (Figure 3a), which may be caused by electron transfer from Ni to Pt, it can be mainly attributed to the Pt−O contribution according to the results of EXAFS analysis (Table 1). On the basis of the above discussion, the promoting behavior of Pt−Ni NPs for MOR can be mainly attributed to “geometric effects”, here referring to surface small NiOx clusters and surface Ni oxide species.

structures as shown in Figure 6. The comparison of the Ni Kedge XANES data collected in 10% H2 and 90% He at 393 K with those collected in air at RT for PtnNi/C are shown in Figure 7. The significant change of Pt3Ni/C compared to both

Figure 7. The comparison of the Ni K-edge XANES data collected in 10% H2 and 90% He at 393 K with those collected in air at RT for PtnNi/C.

4. CONCLUSIONS In the present paper, carbon supported Pt and PtnNi (n = 3, 2, 1) electrocatalysts were obtained by a modified polyol process and were characterized by XRD, TEM, XANES, and EXAFS techniques. In air, the NPs of Pt3Ni/C are demonstrated to have a quasi core−shell structure consisting of a core of metallic Pt surrounded by small NiOx islands. In contrast, both Pt2Ni/C and Pt1Ni/C are demonstrated to have an alloy structure with partial oxidation on the surface. The possible MOR activity for PtnNi/C is discussed according to their structure and the Pt3Ni/C should show best activity. In an atmosphere of 10% H2 and 90% He, the PtnNi/C are expected to become bimetallic alloy. It is found that the Ni amount can influence the structure of Pt−Ni NPs and the Pt3Ni/C is more sensitive to environment.

Pt2Ni/C and Pt1Ni/C suggests that Pt3Ni/C is more sensitive to environmental atmosphere. It is also important to note that the XANES of the reduced Pt3Ni/C at 393 K is very similar to those of Pt2Ni/C and Pt1Ni/C, implying formation of an alloy as a result of cutting off the path to combine with O. A similar phenomenon about inward diffusion of Ni atoms was also observed by reduction of carbon supported Pt−Ni catalysts in H2.62 Our results show that the Pt−Ni bimetallic NPs with less Ni amount are more sensitive to the environment, so it may give some reference information to study the Pt−Ni bimetallic catalysts in the in situ environment. The as-synthesized PtnNi/C catalysts having different structures (especially surface structures) should exhibit varied activity improvement for MOR compared with Pt/C. The possible MOR activity for PtnNi/C is discussed below according to their structures. It was reported that the high reactivity to CO oxidation for a well-defined sandwich Ni− Pt(111) surface can be well illustrated by the synergetic effect of the surface Ni oxide nanoislands and subsurface Ni atoms.61 The Ni oxide species were also considered as a contribution for COads oxidation, thereby enhancing the methanol oxidation current.11 According to Antolini et al.,14 the improved performance of Pt−Ni electrocatalysts as anode material for MOR can be attributed to the presence of nonalloyed NiO species, which may specifically present as NiO, Ni(OH)2, or NiOOH.10 So the oxygenated species occurring in PtnNi/C, such as small NiOx clusters and surface Ni oxides, may be a benefit for removing the COads. On the other hand, methanol oxidation is a slow reaction that requires active multiple sites for adsorption of methanol and sites that can donate OH species for desorption and oxidation of the adsorbed methanol residues.63 Indeed, it is well established that at least three adjacent Pt sites in the proper crystallographic arrangement are necessary to activate the chemisorption of methanol.19,64 However, the Pt surface may be disturbed and changed when the Pt atoms alloy with Ni. Generally, the EXAFS analysis only gives average information, so the NNiO is associated with the proportion of surface Ni relative to whole Ni atoms of the alloy NPs. The bigger NNiO (Table 1) suggests that the Pt1Ni/C have

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ASSOCIATED CONTENT

S Supporting Information *

The k-range for FT and r-range for fitting the EXAFS data: TEM and HRTEM images with corresponding histograms of the particle diameter distributions of the Pt/C and PtnNi/C catalysts show that the NPs of the Pt/C and PtnNi/C are disorderly distributed on carbon support having narrow particle size distributions, and the average size is approximately 3 nm. Comparison results of Ni K-edge XANES data collected in H2/ He at 310 K, 393 K, and in air at RT for the PtnNi/C: EDX line-scanning profiles of nanoparticles of the PtnNi/C show the NPs of Pt3Ni/C have significant different distribution of elements Pt and Ni in comparison with both Pt2Ni/C and Pt1Ni/C. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: (+86)-21-33933212. Tel: (+86)-21-33933212. E-mail: [email protected] (Y.H.); [email protected] (J.W.). Fax: (+86) 411-84379797. Tel: (+86)-411-84379063. E-mail: [email protected] (G.S.). 20589



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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the project of Chinese Academy of Sciences (No. KJCX2-YW-N43), National Natural Science Foundation of China (Grant No. 11175244, 91127001) and External Cooperation Program of the Chinese Academy of Sciences (Grant No. GJHZ1135). We are thankful to the members of the Beam Line BL14B1 and BL14W1 at SSRF for the XRD and XAS beam time.

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Structure of PtnNi Nanoparticles Electrocatalysts Investigated by X-ray

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